Ten Japanese nationalists who planted flags on a remote island have caused a wave of furious protest across China, because the sovereignty of the islands is disputed.
The subject of the rage is five uninhabited islets and three barren rocks, 120 miles northeast of Taiwan.
The islands, known as the Diaoyu Islands in China and the Senkaku Retto in Japan, are technically Japanese - the territory was transferred to the Japanese in 1971 - but China disputes this, claiming the islands have been Chinese for many centuries previously.
An anti-Japan protester holds a poster which reads "Declare War on Japan" in Hangzhou, in eastern China's Zhejiang province
Members of Gambare Nippon, Japanese nationalists, swum ashore from a protest flotilla on Saturday, planting the red and white Rising Sun flag on the island.
They were responding to 14 Hong Kong protesters, who were arrested after trying to plant a Chinese flag on the island earlier this week.
The Chinese protesters will be deported, but will not be prosecuted in Japan, Tokyo has promised.
Tokyo politician Eiji Kosaka told the Agence France Presse: "This is undoubtedly Japanese territory. On the mountain we found Japanese-style houses that had places for drying fish.
"It is very sad that the Japanese government is doing nothing with these islands."
Armed police were called in across China to quell angry anti-Japanese demonstrations.
Crowds of up to 3000 gathered in Shenzhen, Hangzhou, Hong Kong, Guangzhou, Shenyang, Harbin, Qingdao and Chengdu, with Japanese flags set alight and Chinese media said Japanese restaurants and cars had been targeted.
Japanese activists hold the national flags on Uotsuri island, one of the islands of Senkaku in Japanese and Diaoyu in Chinese, in East China Sea
Tensions in the region have been close to boiling point this week, which saw the anniversary of Japan's World War II surrender.
Japan has apparently sought to buy more islands in the disputed area -which currently belong to a private individual, a move China believes is unlawful.
Japanese media reported that the country's ambassador to China is in the process of being replaced, after he criticised the proposal to purchase the islands.
The islands look unimpressive at first glance, but China, Taiwan and Japan all want control of the territory for defensive purposes, and for the rich mineral and oil deposits under them.